A popular conventional type of packing for the stuffing boxes of high pressure, high temperature valves and pumps includes several pieces which are separately made and installed in the stuffing box. Such a construction may, for instance, include one or more axially-central rings made of compressed flexible foamed graphite tape, sandwiched between end rings which prevent extrusion of the graphite around the packing gland or along the stem and out of the stuffing box.
The usual instructions which come with a high temperature, high pressure valve or pump conventionally call for use of a multi-ring stem packing assembly, based on the manufacturer's knowledge of what kinds of packing are available, the problems likely to be encountered in use, and the normal installation procedures with which installation and service personnel can be expected to be familiar.
It is widely thought that high pressure high temperature valves require multiple-ring stuffing box assemblies to provide adequate seals. Braided packing rings, which are widely used, have joints and so two or more of them are usually stacked, with their joints angularly offset in order to eliminate leak paths. Endless rings made of compressed flexible graphite tape, although they are circumferentially continuous, are generally provided as a sandwich construction including one or a plurality of graphite rings sandwiched between anti-extrusion end rings.
In practicing a conventional installation procedure, a bottom braided end ring is installed in the stuffing box and seated. Next, one or more compressed flexible graphite rings are installed (with their radial joints angularly staggered, if they are split rings). Next, the top braided end ring is installed and seated.
Finally, a gland follower is installed and the gland or bonnet is tightened according to specification.